Recently, along with the advance in the information technologies, the multimedia technologies, and the multimedia technologies, there has been an increasing demand for optical information recording media having a higher density and a larger capacity.
An upper limit of a recording density of the optical information recording medium is determined mainly by a spot diameter of a light beam for information recording or reproduction. The reason for this is as follows: As a recording mark on the optical information recording medium is reduced in diameter for increasing a density, a plurality of recording marks are included in a spot region, and consequently the recording marks cannot be detected.
Incidentally, the spot diameter of a light beam is substantially expressed by λ/NA, where λ is a wavelength of light emitted from a light source, and NA is a numerical aperture of an objective lens for forming the optical spot. Therefore, for the optical information recording medium, an effort has been made to substantially improve a recording density by reducing the spot diameter of the light beam by shortening the wavelength λ of light emitted from the light source and increasing the numerical aperture NA of the objective lens.
However, it is considered that the wavelength λ of light emitted from the light source cannot be shortened to below a wavelength in an ultraviolet ray range, due to (i) absorption by an optical element and (ii) restriction given by sensitivity properties of a detector. Further, an improvement in the numerical aperture NA of the objective lens is limited by a tolerance on inclination of an optical axis of a light beam with respect to the optical information recording medium. Therefore, there is a limit on the effort to improve a recording density by reducing the spot diameter of a light beam by changing the NA or the wavelength λ of light emitted from the light source.
Accordingly, an effort has been made to develop an optical information recording medium employing a super-resolution technique, which enables reproduction of information specified by a recording mark having a length equal to or shorter than a diffraction limit (hereinafter, referred to as “equal to or shorter than a light diffraction limit”) of a reproduction optical system. Hereinafter, the optical information recording medium employing this technique is referred to as “super-resolution optical information recording medium”. Also, reproduction of information specified by a recording pit having a recording mark length equal to or shorter than the light diffraction limit which reproduction uses the above technique is referred to as “super-resolution reproduction”.
Generally, it is said that the light diffraction limit of the reproduction optical system is approximately λ/(2NA) (λ: a reproduction light wavelength, NA: an aperture ratio of a lens), due to a restriction given by a frequency limit of a detectable signal.
The “approximately λ/(2NA)” corresponds to a cycle size of a pattern constituted by repetitions of a single-size recording mark and a single-size space, and it is known that half of this, i.e., approximately λ/(4NA) is a resolution limit in terms of a recording mark length. Thus, hereinafter, the resolution limit means λ/(4NA), which is the resolution limit in terms of the recording mark length. Note that an actual resolution limit is determined also by effects given by other elements in the optical system, in addition to the theory; therefore, a value of the resolution limit may deviate to some degree from the theoretical value obtained according to the wavelength and the numerical aperture.
As a technique for exceeding the resolution limit to enable the super-resolution reproduction, there is a super-resolution technique as described in Patent Literature 1.
Patent Literature 1 discloses, as an example of an optical information recording medium employing the above-described super-resolution technique, an optical information recording medium for which information recording or reproduction is performed by emission of laser light, the optical information recording medium including a phase-change recording film, a reflective film, and a carbon thin film provided in the vicinity of the phase-change recording film, the carbon thin film serving as a signal enhancing film.
Further, the optical information recording medium described in Patent Literature 1 employs a system by which recording marks having the same form and each having a length equal to or shorter than the light diffraction limit are positioned along a direction in which signal reproduction is performed. From the optical information recording medium described in Patent Literature 1, information specified by single-frequency repeating phase pits (a mark-to-space ratio of 1 to 1; hereinafter, referred to as “monotone pattern system”) is reproduced. Furthermore, C/N (Carrier to Noise ratio) is employed to evaluate a reproduction property of this optical information recording medium. According to this evaluation, Patent Literature 1 describes the results of examples relating to the super-resolution reproduction.
Additionally, Patent Literature 1 describes that a reproduction laser power for reproducing the super-resolution optical information recording medium needs to be greater than that for reproducing an optical information recording medium which does not need the super-resolution reproduction. Further, Patent Literature 1 also discloses a phenomenon that, as a reproduction laser power increased, phase-change recording marks were broken, and accordingly reproduction signals were reduced. Patent Literature 1 assumes that the breakage of the phase-change recording marks was triggered by the heat. Thus, there is a problem with infinitely increasing a reproduction laser power in order to reproduce information from such the super-resolution optical information recording medium.
Patent Literature 1
Japanese Patent Application Publication, Tokukai, No. 2006-18976 A (Publication Date: Jan. 19, 2006)
Patent Literature 2
Japanese Patent Application Publication, Tokukai, No. 2001-250274 A (Publication Date: Sep. 14, 2001)
Incidentally, optical information recording media generally employ a system (hereinafter, referred to as “random pattern system”) by which a plurality of marks having lengths regularly different from each other are positioned according to a predetermined system along a direction in which signal reproduction is performed, rather than the monotone pattern system disclosed in Patent Literature 1.
The reason for this is that a recording density can be improved more when information is stored according to the random pattern system than when the information is stored according to the monotone pattern system. This also applies in the optical information recording medium employing the super-resolution technique, which enables reproduction of information specified by the recording mark having a length equal to or shorter than the light diffraction limit.
Note that there are a lot of examples of practical application of the random pattern system. For example, CD (Compact Disk) employs the EFM (8-14) (Eight to Fourteen Modulation). DVD (Digital Versatile Disk), Blu-Ray (Registered Trademark) Disc (BD), and HD-DVD employ modulation modes different from that employed by CD. Specifically, DVD employs the EFM Plus (8-16); BD employs the 1-7PP modulation; and HD-DVD employs the ETM (8-12). Thus, a lot of optical information recording media employ the random pattern system which can improve the recording density.
As a result of studies, the inventors of the present invention found the following fact: In reproduction of information from an optical information recording medium which information is stored in the optical information recording medium according to a random pattern system including a recording mark having a length equal to or shorter than the light diffraction limit, improving C/N only does not necessarily provide a favorable reproduction quality, and therefore it is important to evaluate and reduce jitter.
In reproduction of information from an optical information recording medium which information is stored in the optical information recording medium according to the random pattern system, which is a generally-used recording system, a reproduction signal quality is important. The reproduction signal quality can be evaluated by various indexes such as C/N, jitter, a bit error rate, or asymmetry of a reproduction signal waveform.
Reproduction of information from an optical information recording medium is performed in such a manner that a reproduction light beam is emitted onto a recording mark specifying the information, and then a change in an amount of reflected light (reflection intensity) therefrom is detected by a detector, so that a signal is reproduced. Actually, in this process, there occurs a position error in a transition point of a signal, i.e., jitter. The jitter occurs due to factors such as (i) a noise caused by a laser, (ii) crosstalk resulting from diffraction light coming from an adjacent track, and/or (iii) a noise resulting from a media defect. If the jitter becomes greater, this causes a reading error in the reproduction system, thereby making it difficult to perform stable reproduction. In view of this, in order to realize stable super-resolution reproduction and a super-resolution optical information recording medium having a higher density, reduction of jitter is indispensable.
In reproduction of information from an optical information recording medium on which only recording marks each having a recording mark length longer than the light diffraction limit are formed, improving C/N reduces jitter. That is, there is a correlation between C/N and jitter. Therefore, in this case, the reproduction signal quality can be improved only by evaluating and improving C/N.
On the other hand, in a case of the optical information recording medium in which information is stored according to the random pattern system including the recording mark having a length equal to or shorter than the light diffraction limit, improving C/N only does not necessarily reduce jitter. In light of this, the inventors of the present invention found that evaluation of jitter is also necessary to improve a reproduction signal quality.
As a result of further studies, the inventors of the present invention found the following fact: In reproduction of information from the super-resolution optical information recording medium which information is stored according to this random pattern system, jitter greatly depends on a reproduction laser power.
In reproduction of information from an optical information recording medium which information is stored in the optical information recording medium according to a conventional random pattern system generally including pits each equal to or longer than the diffraction limit of the reproduction optical system, jitter occurring while the information is reproduced from the optical information recording medium does not depend on a laser power applied thereto. Therefore, in this case, prior to reproduction of content in the optical information recording medium, no special process (hereinafter, referred to as “test reading system”) is required which sets a reproduction laser power for optimizing a reproduction signal quality such as jitter.
However, it was found that, in reproduction of information from the super-resolution optical information recording medium which information is stored according to the random pattern system, jitter greatly depends on a reproduction laser power.
In other words, prior to reproduction of content of the optical information recording medium, such the system is required which sets the reproduction laser power for optimizing a reproduction signal quality such as jitter. Further, the problem described in the above-described Patent Literature 1, i.e., the problem that reproduction signals are reduced as a reproduction laser power increases also applies in the super-resolution optical information recording medium in which the information is stored according to the random pattern system; therefore, also from the viewpoint of solving this problem, it is strongly demanded to set an optimum reproduction laser power for reproducing information from the super-resolution optical information recording medium.